Combining inverters for harmonic reduction

ABSTRACT

A plurality of polyphase inverters are connected in circuit to substantially common polyphase loads through a polyphase reactor. Windings from each phase of each inverter are provided on the reactor to cancel the component of flux of the fundamental frequency within the reactor and to have the reactor substantially absorb the NP + OR - 1 harmonics where N is the number of inverters and P is the number of phases. For two threephase inverters this substantially cancels the fifth and seventh harmonic voltages within the reactor applying a true 12-step voltage wave to each of the two common three-phase loads with nothing smaller than the 11th harmonic voltage supplied to each load. The two inverters are phase displaced by 360*/2NP which normally at a 30* phase angle would be a voltage loss of 3 1/2 percent applied to the load yet this reactor connection applies 100 percent voltage to the loads and the load power factor is reflected directly to each inverter. The loads may be DC isolated, permitting series connection of the inputs of the inverters from a single high voltage DC supply.

United States Patent [1 1 Meier Feb. 12, I974 [75] lnventorz Udo H.Meier, Luzern, Switzerland [73] Assignee: Reliance Electric Company,

Cleveland, Ohio 22 Filed: oc:.12,1971 211 Appl.No.: 187,974

52 us. Cl 307/58, 307/32, 307/82, 318/227, 321/9 R, 321/27 MS, 32l/D1G.1 51 Int. Cl. ..H02j,1-102m [58] Field of Search32l/9 R, 9 H, 26, 27 R,27 MS,

32l/DIG. l; 307/32, 58, 82, 18, 19; 323/83;

OTHER PUBLICATIONS Electrical Manufacturing, Design Techniques forStatic Inverters, pp. 90, 91, Feb. 1960,

Primary Examiner-William H. Beha, Jr. Attorney, Agent, or FirmWoodling,Krost, Granger & Rust 57 ABSTRACT A plurality of polyphase inverters areconnected in circuit to substantially common polyphase loads through apolyphase reactor. Windings from each phase of each inverter areprovided on the reactor to cancel the component of flux of thefundamental frequency within the reactor and to have the reactorsubstantially absorb the NPil harmonics where N is the number ofinverters and P is the number of phases. For two three-phase invertersthis substantially cancels the fifth and seventh harmonic voltageswithin the reactor applying a true 12-step voltage wave to each of thetwo common three-phase loads with nothing smaller than the 11th harmonicvoltage supplied to each load. The two inverters are phase displaced by360/2NP which normally at a 30 phase angle would be a voltage loss of 3/2 percent applied to the load yet this reactor connection applies 100percent voltage to the loads and the load power factor is reflecteddirectly to each inverter. The loads may be DC isolated, permittingseries connection of the inputs of the inverters from a single highvoltage DC supply.

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Fig, 17 U00 5%??? COMBINING INVERTERS FOR HARMONIC REDUCTION BACKGROUNDOF THE INVENTION The art of building inverters has been developed overthe past few years until now there is need for high powered multipleinverter systems. Often the systems feed induction motor loads atvariable voltage and frequency and must operate over a wide range.

It is well known to output an inverter into a single or polyphaseisolating transformer and to combine windings therefrom to takeadvantage of phase displacements in the inverters tocancel harmonics.This technique while straight-forward requires that each transformerhave the volt second capacity materialwise to handle the minimuminverter frequency at maximum voltage. For low frequency units thistechnique can result in very large transformers and the practice isalmost limited entirely to fixed frequency inverter systems. 1

For variable frequency systems pulse width modulation is often used toreduce voltage when frequency is down. This allows one to use theisolating transformer technique but still a largema'gnetic structure isrequired particularly when the final wave is made up of many windingsegments at different angles from each other.

A common technique for paralleling two inverters is to use a commoncentertapped reactor, often calling a spanning reactor, between the twoinverters and to take the output voltage as the average of the twoinverter voltages. This reactor is at least not a completely isolatedwinding structure and so is less bulky much as an auto-transformer wouldbe as compared to an isolating transformer. The output voltage of thistype of system removes some of the fifth and seventh harmonic found withthe six-step output from the three phase inverter inputs but does notcompletely get rid of them. The wave shape formed is an imperfectl2-step wave rather than a true or zero fifth/seventh harmonic waveform.

The main objective of paralleling two or more inverters is to obtainhigher power capabilities. This can be realized by connecting twoinverters in parallel with very small reactors of the spanning reactortype, that limit currents between the inverters due to small differencesin timing between corresponding phases. Additionally, with a spanningreactor, the load voltage waveform can be substantially improved byproperly phase shifting the output of the two inverters. In the case ofthe spanning reactor type of FIG. 1, with two three-phase inverters anda single load, then the sixstep waveform from each inverter can bechanged into an imperfect l2-step by phasing the two inverters 30 apart.This is an imperfect l2-step wave because it does contain some fifth andseventh harmonics.

In the prior art spanning reactor arrangement of FIG. 1 the fluxrequired to be carried by the reactor is quite high; hence, the physicalsize of the reactor must be large. The reactor current is equal to theinverter current but the power factor of the two inverters aredifferent. The angular difference in the power factors is equal to thephase displacement between the two inverters. Also, three reactor coresare needed each carphase reactor.

A prior art modification of the spanning reactor is shown in FIG. 2 witha series reactor between one inverter and a load which load also issupplied from another inverter pha se-shifted with respect to the firstinverter.

It is possible to obtain a l2-step output waveform from two three-phaseinverters which each have a sixstep output. Each inverter output issupplied to primaries of different isolating transformers. Thesecondaries of these two transformers are connected in zig-zag fashionin series to obtain a l2-step output wave. However, in such case theisolating transformer has to have a volt-ampere capacity equal to theoutput of the inverter because it must handle all of the outputincluding the fundamental flux component as well as the harmonics.

Accordingly, an object of the invention is to obviate theabove-mentioned disadvantages.

Another object of the invention is to provide a reactor in circuit withparalleled inverters where the reactor is three to five times smallerthan formerly required.

Another object of the invention is to provide an inverter parallelingcircuit utilizing a reactor wherein the fundamental flux component issubstantially canceled within the reactor so that it is physicallysmall.

Another object of the invention is to provide an inverter parallelingreactor which eliminates the 3 /2 percent loss of voltage normallyencountered by paralleling inverters having a 30 phase displacement.

Another object of the invention is to provide an inverter parallelingcircuit with a reactor which substanllxalzs rhs the NP. harrlq s Nbsiaathanymb er ofinverters and P the number of phases, so thatsubstantially no harmonics less than 2 NP: 1 are supplied to the load. 7I W D Another object of the invention is to provide an inverterparalleling circuit wherein the load power factor is reflected directlyto each inverter.

Another object of the invention is to provide two three-phase invertersconnected in parallel wherein the two six-step outputs of the invertersare interconnected to supply two l2-step voltage waveforms to each load.

Another object of the invention is to provide an inverter parallelingcircuit wherein the loads may be DC isolated, thus permitting theinverter inputs to be interconnected such as connected in series.

Another object of the invention is to provide an inverter parallelingcircuit which substantially eliminates the fifth and seventh harmonicvoltages in a common load of the inverters.

Another object of the invention is to provide a reactor in which thevolt second requirement is only a small fraction of the volt secondrequirement of an inverter coupling system using isolating transformers.

Another object of the invention is to show a reactor coupling means inwhich the fundamental frequency is canceled out in the reactor iron andwhich need be designed to support the only harmonic voltage differences.

Another object of the invention is to provide a means of connectingmultiple induction motors to power a common load while being providedwith power from a variable frequency inverter.

Another object of the invention is to show how harmonies can be removedby suitable paralleling of inverters through a reactor havingsubstantially two windings per phase" of inverter.

Another objectof the invention is to transform the square wave outputvoltages of multiple three-phase inverters so that they are more fullyutilized than with other schemes.

SUMMARY OF THE INVENTION The invention may be incorporated in aninverter circuit for supplying energy to a plurality of loads,comprising in combination, a plurality of polyphase inverters operableat a phase displacement therebetween of 360/2NP, where N is the numberof inverters and P is the number of phases, reactor means having Pphases, winding means on said reactor means for each phase and for eachinverter, and means connecting said winding means in circuit with eachinverter phase and its respective load to cancel the fundamentalfrequency component of the flow within said reactor means.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawing.

BRIEF DESCRIPTION OF- THE DRAWING FIGS. 1 and 2 are schematic diagramsof prior art arrangements of inverter circuits;

FIG. 3 is a schematic diagram of a single inverter used in the presentinvention;

FIG. 4 is a schematic diagram of the preferred embodiment of invertercircuit;

FIGS. 5 6 and 7 are schematic diagrams of modifications of theinvention;

FIG. 8 is a graph of voltage waveforms;

FIG. 9 is an ampere turn vector diagram;

FIGS. 10-13 are vector diagrams of harmonic ampere turns; I

FIGS. 14 and'lS are vector'diagrams of fundamental ampere turns; isvoltage and current vector diagrams of two unbalanced loads; and,

FIG. 17 is a vector diagram of voltages.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 illustrates some elementsof a schematic diagram of a single inverter 11 of the polyphase type, inthis case three-phase, supplying a three-phase load 12 from positive andnegative supply terminals 13 and 14. These supply terminals may comefrom any suitable DC source 15, which may be, for example, a threephaserectifier rectifying a three-phase alternating current voltage. Itshould be recognized that there are many types of three-phase inverters,any one of which can provide an input to the reactor windings. Theinverter 11 is a three-phase bridge configuration including three pairsof switches, each pair being connected in series between the positiveand negative terminals 13 and I4. Switches 17, 19 and 21 are connectedto the positive terminal 13 and the switches 18, and 22 are connected tothe negative terminal 14. The junctions 23, 24 and 25 between the threepairs of thyristor switches are connected to supply the three-phase load12.

Each of the load terminal junctions may have only two different voltageconditions. Junction 23 will be positive when switch 17 is closed andwill be negative when switch 18 is closed. The same is true'for theother load terminals. Suitable control circuits are provided tosequentially close the switches to apply an alternating voltage to theload. The inverter may have some means to change the output fundamentalfrequency and may have some means to change the effective output volt-FIG. 3 is a symbolic representation of a three-phase square waveforminverter, which in reality, is nothing but a combination of controllableswitches. Usually, today, the switches are in the form of thyristors anddiodes with suitable gating and turn-off circuitry. Since this inventionis universal, it is not deemed necessary to show more than the symbolicrepresentation for the inverter.

The inverter control may employ any of a number of suitable invertercontrol circuits from simple control circuits up to more complex onessuch as that in the Pulse Width Modulated Inverter", US. Pat. No.3,461,373 issued Aug. 12, 1969; An Integral Ratio Carrier Inverter,application Ser. No. 2,780 filed Jan. 14, I970; or the Pulse WidthModulated Inverter Adaptive Logic, application Ser. No. 77,108 filedOct. l, 1970..

FIG. 4 shows the preferred embodiment of an inverter circuit 31 used forparalleling a plurality of inverters, in this case, two inverters 32 and33, each of which may be of the type shown in FIG. 3. This invertercircuit 31 supplies energy to a plurality of loads and in this preferredembodiment the loads are substantially common and are shown as first andsecond stator windings 34 and 35 each supplying a rotating magneticfield to a common rotor 36, thus establishing the common load. Theplurality of inverters 32 and 33 are operable at a phase displacementtherebetween of 360/2NP, where N is the number of inverters and P is thenumber of phases. In this case there are two inverters and each is athree-phase inverter; hence, the phase displacement is 30 electricaldegrees between the two inverters. Also included in the inverter circuit31 is reactor means. In this preferred embodiment the reactor means is asingle polyphase reactor with the number of phases corresponding to thatof the inverters and the loads. The reactor 37 has a single magneticcore of the threelegged type and this may be a simple flat core such asmade up from a stack of E-I laminations with winding means 39 on each ofthe three legs of this core 38. The winding means 39 are provided on thereactor 37 for each phase and for each inverter. In this preferredembodiment there are two major windings and two minor windings for eachleg 41, 42 and 43 of the reactor core 38.

The winding means are connected in circuit with each inverter phase andits respective load to cancel the fundamental frequency component of theflux within the reactor core 38. In the preferred embodiment this isaccomplished by the winding means 39 having four windings on eachreactor leg, two major windings and two minor windings. Morespecifically the winding means 39 includes major windings 44, 45 and 46connected in series between the inverter 32 and load 34 and includeminor windings 47 49 also connected in series between the inverter 32and its respective load 34. The winding means 39 further include majorwindings 51, 52 and 53 and minor windings 54, 55 and 56 connected inseries between the inverter 33 and its respective load 35. The threeoutputs from the inverter 32 have been designated as 0, and 240supplying respectively the phase windings 57, 58 and 59 of the motorstator winding 34. As stated above, the

inverter 33 has a 30 phase displacement relative to the output ofinverter 32vand in this case is shown as a 30 so that the outputterminals of this inverter 33'are designated as 30, 90 and 210. Theseoutputs supply phase windings 61, 62 and 63 of the second stator winding35, respectively.

FIG. 8 has waveforms 66 and 67 illustrating the output of the inverters32 and 33, respectively. Each is shown as a six-step output voltage wavewhich is the case with an unmodulated output of the phase-toneutralvoltage. By unmodulated is meant when no pulse width modulation ispresent to create notches in the output. These waveforms 66 and 67clearly show the six steps per 360 cycle of the fundamental voltage andshow that the output 67 of inverter 33 is phase displaced 30 Iaggingrelative to that of the output 66 of inverter 32. A review of FIG. 4will illustrate that the line-to-line voltage, that is, the outputterminals of inverter 32, passes through two major windings, two minorwindings, and two load phase windings in series. This connection of thewinding means 39 establishes a true l2-step motor voltage winding asshown by waveform 68 in FIG. 8. Because of the interaction of the fluxon the reactor core 38, each inverter 32 and 33 applies a true 12-stepvoltage to its respective load stator winding 34 and 35. By a truel2-step voltage is meant one which has the proper amplitude of steps sothat there are substantially zero harmonic components of the form (61-1M 12), where =0, l, 2, 3. This has been verified by oscilloscopereadings taken on an inverter system constructed in accordance with thepresent invention.

Waveform 69 of FIG. 8 illustrates the reactor voltage which appearsacross the reactor windings in a particular motor load circuit and isobtained graphically by subtracting the waveform-68 from the waveform66. The voltages shown on FIG 8 are by way of example for an operatinginverter circuit 31. It will be observed from the waveform 69 that thereis no fundamental component of voltage present in the reactor 37, onlyhigher harmonics and primarily the fifth and seventh harmonics of thevoltage; The fifth and seventh harmonics have been neutralized andsubstantially canceled from the l2-step voltage waveform 68 and appearsubstantially only across the reactor 37.

FIG. 9 gives a vector diagram of the ampere turn relationships in thereactor 37 The vectors of the ampere turns on FIG. 9 have been given thesuffix A to the reference numerals 44-56 to indicate that they are theampere turns caused by the respective winding 44-56. These ampere turnvectors 44A-56A are those at the fundamental frequency of operation ofthe inverter circuit 31. Referring to the vector diagram of FIG. 9,three lobes are shown, one for each phase.

The vector 51A, for example, is 150 displaced from the vector 44A inthis FIG. 9 because of the connection in opposition of the windings 44and 51 relative to the inverter voltage sources 32 and 33. Accordingly,a negative vector 51A has been drawn as a part of the first lobe andother vectors have been designated as negative vectors because of theconnection in opposition of the respective windings. One notes thatvectors 44A and 47A may be combined to produce a vector 71 horizontallyto the right in this figure. Also vectors 54A and SlA may be combined toproduce a vector 72 horizontally to the left. The net result of thesetwo vectors 71 and 72 is zero which is another way of stating that thenet results of the vectors 44A, 47A, 54A and S-lA is zero. The sameistrue for each of the other two lobes of this vector diagram and thisillustrates that the resultant ampere turns are zero for the 30phasesh'ifted fundamental currents.

To achieve this zero ampere turns of the fundamental, each of the minorwindings 47 -49 and 54-56 are a value of K times the ampere turns of themajor windings 44-46 and 51-53. In this case the factor K equals f 3 l)/2 0.366 This value may be obtained either by geometry from FIG. 9 orcalculated mathematically. Further, from FIG. 9 it will be noted thatthe ampere turn vector 47A is parallel to but in the opposite directionfrom ampere turn vector 46A. This is chosen purposely so that theseampere turn vectors may be obtained from the actual reactor 37. As shownin FIG. 4'

the windings 46 and 47 are in series so that they have the same currenttherethrough and the same phase angle. The schematic diagram of FIG. 4illustrates a dot beside each winding and this indicates the start ofthat particular winding. This shows that the flux established bywindings 44 and 51 are generally in opposition;

namely, out of phase as illustrated in FIG. 9. This zero resultant ofampere turns for the fundamental currents in the reactor has twobeneficial results. All of the fundamental component is passed to themotor or other load in order to supply torque or power thereto andsecondly, the volt-second capacity or volt-ampere capacity of thereactor 37 may be made much smaller than normal. It may be made muchsmaller because now the reactor need only support the flux of the fifth,seventh, seventeenth, nineteenth and higher harmonics rather than havingto support the flux of the fundamental as in the prior art designs ofreactors or isolating transformers.

FIG; 10 is a vector diagram of the ampere turns of the fifth harmoniccurrents in the reactor 37. In this case the ampere turn vectors on FIG.10 have been given a subscript-of Ssuch as 44,, to indicate the fifthharmonic ofthe ampere turns for each of the windings 44-56. For thefifth harmonic, this is a backward traveling harmonic relative to thefundamental, which in a motor, establishes a retarding torque on themotor. Accordingly, the main vectors 44 45 and 46 are shown as beingdisposed in a counterclockwise rotational direction. The 30 displacementbetween the inverter outputs ofthe fundamental results in a five timesthat angle or 150 displacement between the fifth harmonic of theinverter outputs. The fifth harmonic vectors for the second inverter 33are accordingly displaced 150 relative to the vectors of the firstinverter 32 in this FIG.

In FIG. 10 the vector 51 has been shown also the negative thereof -51,as a part of the lobe of this vector diagram which extends to the right.Again, as in FIG. 9 this is because the windings 44 and 51 are connectedgenerally in opposition relative to the inverters 32 and 33.-FIG. 10shows that there are four vectors 44 47 Sl and 54 in one lobe which givea resultant ampere turn vector 73 for this fifth harmonic. There are twoother symmetrically spaced vectors not shown on the FIG. 10, for theother two phase-to-neutral voltages, but similar to the three lobes ofFIG. 9. This illustrates that there is a large resultant ampere turnvector for the fifth harmonic, the vector 73, present in this reactor37. Also, this FIG. 10 illustrates that substantially none of the fifthharmonic voltage from each of these two inverters 32 and 33 is passed tothe loads 34, 35. This again is very desirable because for a motor loadthe fifth harmonic is a backward rotating or retarding torque harmonicand for all loads it is merely something which creates additionalheating. By having this fifth harmonic largely present within thereactor 37, a superior voltage wave 68, FIG. 8 is supplied to the load34, 35.

FIG. 11 is similar to FIG. 10 but is a vector diagram of the ampere turnvectors for the seventh harmonic within the reactor 37. Again thevectors have been provided with a subscript 7 for each of the windings44-56. The seventh harmonic is a forward rotating vector relative to thefundamental. Also, for this seventh harmonic the vector.51 is displaced210 or seven times 30 relative to the ampere turn vector 44,. Again inFIG. 11, as in FIG. l0, only a single lobe has been shown, however theother two lobes symmetrically displaced are present. The lobes shown inFIG. 11 shows a resultant ampere turn vector 74 for the seventhharmonic. This illustrates that a large seventh harmonic flux is presentin the reactor 37 and it substantially cancels these seventh harmonicsfrom being applied to the two loads 34 and 35. FIG. 12 is a vectordiagram of the ampere turns for I the 11th harmonic within the reactor37. This 11th harmonic for each of the windings is illustrated by asubscript 11 for the numerals 44-56. The 11th harmonic 7 is a backwardrotating vector and is phase-shifted eleven times 30 or 330 between theinverter outputs as illustrated in this FIG. 12. Again considering onlyone lobe, or one reactor leg 41, the ampere turn vectors 44,, and 47combine to form a vector 75. Ampere turn vectors 54 and 5l' combine toform a vector 76 which is equal and opposite to the vector 75. Thisshows that the resultant ampere turns equal zero for this 11th harmonicin the reactor 37. The same is true for the other two legs 42 and 43although not shown in this FIG. 12.

FIG. 13 shows the ampere turn vectors witha subscript 13 for the 13thharmonic within the reactor 37. For this 13th harmonic there is a 390displacement between the l3th harmonic output of the two inverters.Also, for this 13th harmonic this is a forward rotating vector rotationrelative to the fundamental. Again in FIG. 13 only one reactor core leg41 is illustrated and the vectors 44, and 47, combine to produce avector 77. Vectors 54, and 5l, combine to produce a vector 78 equal andopposite to vector 77 to show that the resultant ampere turns equal zerofor this 13th harmonic within the reactor 37. This means that the I 1thand l3th harmonics are not supported within the reactor 37, no resultantflux at these harmonics occurs within the reactors, and hence these arepassed to the load 34, 35. Accordingly, referring to FIG. 8, none of the11th and 13th harmonics appear in the reactor voltage waveform 69 yetthese llth and 13th harmonic voltages do appear in the motor voltagewaveform 68.

FIG. 5 is a modification of the invention and quite similar to theschematic diagram of FIG. 4. FIG. 5 shows a schematic diagram of aninverter circuit 81 wherein the reactor 37 may be identical to that inthe circuit of FIG. 4. Inverter 32 and 33 may also be identical. Theload for the inverter circuit 81 is shown as a common load 82 for twoseparate motors 84 and 85. Each motor has a three-phase stator winding86 and 87, respectively, energized from the inverters 32 and 33,

respectively. Separate rotors 88 and 89 cooperate with the statorwindings 86 and 87. The two rotors 88 and 89 may be connected on acommon shaft or on a common load or substantially common load, forexample, two separate motors driving two separate axles on aself-propelled railway car. Despite slight variations in diameter of thewheels on these two axles, this may be considered a substantially commonload. It will be noted that in FIGS. 4 and 5 there is no DC connectionbetween the two stator windings in each inverter circuit. In otherwords, the loads are DC isolated and this permits considerably moreflexibility in the energization arrangements of the inverters 32 and 33.In FIG. 5 the inverters are shown as connected in series across thepositive and negative terminals 13 and 14 so that these two invertersmay be energized from a single high-voltage DC source. If the two loadswere DC interconnected as in the prior art arrangements, then this wouldestablish circulating currents or DC currents in the loads which couldcause overheating or could otherwise be damaging. The present inventionpermits this electrical interconnection of the inputs 32 and 33. Theymay also be connected in parallel, because of the DC isolation of theloads. The vector diagrams of FIGS. 9-13 also apply to the circuit ofFIG. 5.

In FIG. 5 the load stator windings 86 and 87 are shown as physicallydisplaced thirty electrical degrees, just as in FIG. 4. In, the case oftwo physically separate electric motors with two separate rotors, it isnot necessary to establish this 30 phase displacement between the statorwindings, because each stator winding cooperates with a separatesquirrel cage rotor which is homogeneous around its periphery. The 30phase displacement of the loads then occurs naturally because of the 30phase displacement between the inverters 32 and 33.

FIG. 6 is a further modification of the invention showing an invertercircuit 91. Inverters 32 and 33 may be the same as in FIGS. 4 and 5supplying substantially common loads 92 and 93., respectively, of thesame number of phases as the inverters. These loads may be motor loadsas in the previous schematic diagrams or may be other polyphase I loadswhich are interconnected or are substantially common. The invertercircuit 91 includes a reactor 97 again on a single core 98 which may bea three-legged core, the same as the core 38 for FIG. 4. This reactor 97has been slightly simplified over the reactor 37by having fewer windingsin the winding means 99. The winding means 99 includes major windings101, 102 and 103 connected in series between the output of the inverter32 and the load 92. The winding means 99 also includes minor windings104-109 connected in series in pairs between the individual phaseoutputs of inverter 33 and the phases of the load 93.

In FIG. 6 it will be noted that from the output of the inverter 32 thereare two reactor windings 101 and 102 in series with two separate phasewindings of the load 92. Adding the ampere turn vectors from each of thewindings on a particular leg of the reactor gives canceling flux vectorsfor the fundamental frequency component. From two of the outputterminals of inverter 33 there are four of the minor windings 104, 105,106 and 108 in series with two of the phase windings of the load 93.Each of the minor windings has fewer turns than the major windings by afactor K and in this case the factor K equals H3 or 0.577. FIG. 14illustrates the ampere turn vectors of the fundamental available fromthe inverter circuit 91 of FIG. 6. On this FIG. 14 a suffix A hasbeenplaced for the numerals 101 through 109 to indicate the ampere turnvectors for the correspondin'gly numbered winding 101-109. On the Zerodegree phase leg of the reactor 97, major winding 101 is placed and alsominor windings 104 and 105. The vector sum of these two ampere turnvectors 104A and 105A is the vector 111 which cancels the ampere turnvector 101A. The same is true for each of the other two reactor legs,the fundamental ampere turns are canceled for a zero resultantampereturns of the fundamental flux component in each leg of the reactor 97.Each reactor leg can be analyzed for fifth and seventh harmonic showingthis arrangement too will cause their fluxes to add similarly to FIGS.and 11. Thus this establishes the same result as the reactor 37 in FIGS.4

and 5.

FIG. 7 is a schematic diagram of another embodiment of the inventionshowing an inverter circuit 121 again operating from the same inverters32 and 33 and supplying energization to a substantially common load 122.This load may take the form of an electric motor having stator windings123 and 124 supplying rotating torque to a common rotor 125. The statorwindings 123 and 124 are shown as three-phase with an open neutralconnection, neaning six terminals for each stator winding. The statorwinding 123 has three separate phase windings 138, 139 and 140 and thestator winding 124 has three separate phase windings 141, 142 and 143.

A reactor 127 is used in the inverter circuit 121 and this reactor has areactor core 128, a single core with three legs the same as reactor 37.Winding means 129 are provided on the reactor 127 and this winding meanshas been simplified over that shown in FIG. 4 by having only sixwindings 131-1'37. It will be noted that the inverter circuit 121 has aseries connection of windings on the reactor plus phase windings in themotor load between two output terminals of the inverter, the same as allof the other inverter circuits 31, 81 and 91. In this case, however, theload 122 is placed between the inverters and the reactor 127. Againthere is a cancellation of the fundamental component of flux in thereactor 127 and in this case of FIG. 7 it is accomplished by a delta-wyeconnection of the windings 131-137 on the reactor 127. The windings131-133 may have a unity value and these are connected in Wye. Thewindings 134-136 have a greater number of turns by a K factor, in thiscase, the square root of 3 or 1.732. These windings 134-136 areconnected in delta. FIG. shows a vector diagram of the fundamental fluxcomponent ampere turns of the reactor 127. On this vector diagram thesubscript A again has been used to designate the ampere turns for thecorrespondingly numbered winding 131-136. It will be noted on FIG. 15that the vector 131A is equal to and opposite to the vector 134A showingthat the fundamental component of flux or ampere turns is canceled inthat particular leg of the reactor core 128. Also, vector 133A is equalto and opposite to the vector 136A for cancellation of the fundamentalflux in that leg. On this FIG. 15 there are vectors 134B, 1358 and 1368.Because the windings 136 have more turns by a factor of K, which in thiscase is 1.732, it might at first appear that the ampere turn vectorsshould be those shown by the suffix B. However, it will be appreciatedthat if the winding 131 has 100 turns and carries one unit of currentbecause of its wye connection, this balances and cancels the ampereturns from winding 134 which has 173 turns and 0.577 units of current.

FIG. 16 illustrates vectors of voltages and current for the circuit ofFIG. 5, for example, where two separate three-phase motors are shown andthe load on each might be different. In FIG. 16 the suffix B has beenused on various numerals to indicate voltages for that particularreactor winding or motor winding. Only one phase of the inverter circuit81 is shown with the zero phase supplying current to phase winding 57 ofthe stator winding 86 and the inverter 30 output of inverter 33supplying current to the phase winding 61 of the stator winding 87. Thevector R is the phase-to-neutral of the first inverter 32 and the vectorU is the phase-toneutral voltage of the second inverter 33. Vector V,,is the total reactor voltage in reactor 37 due to the first inverter 32.V is the total reactor voltage in reactor 37 due to the second inverter33. a is the phase angle of the first motor 86 and a is the phase angleof the second motor 87. i, is the motor current in the first motor phasewinding 57 and i,,. is the motor current in the second motor phasewinding 61.

FIG. 16 shows that with different loads on the two motors, the reactor37 still forces the motor currents to be equal and 30 phase displaced.The reactor takes fundamental voltage away from the more loaded motorand adds voltage to the other one. The power difference between the twomotors is obtained mainly through the change in motor power factors. Itwill be noted that the motor with the larger voltage, vector 578, has amore lagging current i, at a larger phase angle a,. In this FIG. 16 itis assumed that the second motor is the more heavily loaded and hencethis FIG. 16 shows that the motor phase winding 61 has a voltage vector618 which is reduced relative to the vector 578, the voltage applied tophase winding 57. This arrangement has a tendency to equalize poweroutput of the motors with the voltage and hence the load being raised onthe less loaded motor. This is true so long as the slip differencebetween the motors remains small. For induction motors this is withinthree to five percent of motor speed, for example. A system such as thisinverter circuit 81 can operate under slip control or it can operateunder open loop when the motors are mechanically rigidly coupled.

This FIG. 16 also helps to illustrate that the power factor of the loadis reflected directly to each inverter. This is not the case with theprior art spanning reactor of FIG. 1 or the series reactor of FIG. 2.

FIG. 17 illustrates another advantage of the present invention. For theprior art spanning reactor of FIG. 1 the two inverters 32 and 33 operateat a 30 phase displacement. Three separate cores for three separatespanning reactors 151, 152 and 153 are provided and the center taps aretaken to the three-phase load 154.

In FIG. 17, vector 156 is the output voltage vector of one phase ofinverter 32 and vector 157 is the vector of output voltage of one phaseof inverter 33 of the FIG. 1 circuit. The resultant voltage vector 158supplied to the load 154 is accordingly the vector sum of vectors 156and 157. One-half of this or vector 159 is effectively the voltagesupplied by each inverter to the load 154. Due to the 15 angle betweenvectors 156 and 159 andsince the cosine of l5 equals 0.966, this showsthat the effective voltage supplied to the load by the prior art FIG. 1is equal to vector 159 whereas the 1 1 effective voltage supplied to theload by the presentinv'ention Circuit of FIG. 4 is vector 156. Thisvector is about 3 /2 percent longer, meaning 3 V2 percent more voltageis supplied to the load making the combined system more effective inutilizing inverter capacity. A comparison of the reactors of the presentinvention and those of the prior art FIGS. 1 and 2 for a particular widefrequency range application shows that the present invention requires amuch smaller reactor physically. Reactor weight can be compared by thefollowing formula: I I

V, the highest value of volt-seconds of the reactor flux occurring atany point of the voltage/frequency curve of the inverter.

i per unit current through the reactor windings,

with unity being the full load AC current of one inverter.

L the number of coils linked to the same flux.

C the number of cores (a three-legged core is rated In the prior artspanning reactor configuration of FIG. 1, the motor voltage is animperfect l2-step waveform that contains Spercent of the fifth harmonicand 4 percent of'the seventh harmonic. The line-to-line motor voltage is460 volts for a 600 volt DC input to the inverters. The-motor current istwice the reactor current. Thereactorvoltage consists of a fundamentalcomponent that increases with phase shift between the inverters as wellas all orders of odd harmonics. The highest flux level of 2.45volt-seconds occurs at the lower frequency limit of 1 Hz. The reactorcurrent is equal to the inverter current. The power factor of the twoinverters are different, the angular difference is equal toithe phaseshift of the inverters. Three separate reactor cores are needed, eachcarrying one winding. The weight factor therefore amounts to:

FIG. 2 illustrates another prior art inverter circuit utilizing a seriesreactor connection. Again the two inverters 32 and 33 are displaced, inthis case by 150. A series reactor 161 has a single core with threeseparate open neutral windings 162, 163 and 164 thereon. A three-phaseload 165 shown as a motor, one which has three separate open neutralphase windings connected in series with the reactor windings 162-164,respectively, and connected between the two inverters 32 and 33. In thisconfiguration the motor voltage across one winding is twice as high asthe phase-to-neutral voltage in the spanning reactor arrangement ofFIG. 1. The voltage waveforms are identical to those of FIG. 1. Themotor current is equal to the inverter and reactor current. The reactorvoltage consists of all in-phase harmonies inherent in the combinedinverter voltages. The highest flux of 1.29 volt-seconds is induced atthe lowest frequency. The two inverters operate at different powerfactors, the angles differ by the phase shift angle between theinverters. The weight factor, therefore,

amounts to:

This series reactor prior art arrangement has the defect thatdifferences of potential, for instance at the positive bus input to theinverter, can cause a DC current to flow through all three phases thattends to saturate the reactor.

The FIGS. 4-7 show various arrangements of the present invention.Considering the circuit of FIG. 4, for the moment, one notes that themotor current is equal to the reactor and inverter currents. The highestflux level occurs at 1 Hz (more voltage is required at low frequenciesto compensate for motor resistance losses) and is equal to 0.23 V Thepower factor of both inverters is equal when the phase shift between theinverters is 30. The reactor 37 has a core 38 which is a three-leggedreactor core. Four coils are on each reactor leg, two coils are majorwindings with a given number of turns, and two coils are minor windingseach with a K factor of 0.366 times that given number. This adds up to2.73 effective number of coils linked to the flux on each leg. Theweight factor then becomes:

A comparison of Formulas 2, 3 and 4 show that the present invention isfour to eight times better than the prior art with respect to thephysical size and weight of the reactor. The reason for this is that thereactor 37 does not need to support any of the fundamental fluxcomponent, the smallest harmonic component of flux which it supports isthe fifth harmonic. Accordingly, the reactor may be made much smallerand lighter for a saving in manufacturing costs. If load unbalance isexpected, some increase in reactor size may be required.

In the present invention inverter circuits of FIG. 4-7 it is shown thatthere is no DC connection between the two loads and hence it is possibleto interconnect the DC input sides of the two inverters. This isillustrated in FIG. 5 wherein the inputs to the two inverters 32 and 33are connected in series from a high voltage DC power supply. 7

Another advantage of the inverter circuits of FIGS. 4-7 is that each ofthe two inverters supplies a 12-step output voltage waveform to each ofthe two loads. Each inverter singly has a six-step output voltagewaveform but by being combined in the reactor 37 with flux from theother inverter, this provides a 12-step output voltage waveform fromeach inverter and reactor set.

It will be noted from FIGS. 4-7 that one inverter has a zero degreesphase angle and the other inverter has a 30 phase angle. This secondinverter may be oper ated at a phase angle which is from that shown. Inso doing, the starts of the various reactor windings for the secondinverter would also be inverted, and as a result, the resultant ampereturn vectors would still be the same as shown in FIG. 9. The 30 phasedisplacement is thus electrically equivalent to a 150 or a 210 phasedisplacement, with appropriate reversal of the connections to thereactor windings. The phase displacement between the inverters hasaccordingly been described as 360/2NP as a minimum phase displacement,and this is to be construed to cover 150 or 210 phase displacement also.

The inverter circuits of the present invention shown in FIGS. 4-7eliminate or greatly minimize the harmonics supplied to the load.Harmonics of the order of 5 M i 12 and 7 +M 12, where M equals 0, 1, 2.,are of the order of harmonics which are minimized by this invention. Asshown in FIG. 10 and 11, there is a large fifth order and seventh orderharmonic flux in the reactor for a minimum voltage of these harmonicsapplied to the motor. The same is true for the 17th and 19th harmonics,35th and 37th and the like. It is true that slight inaccuracies orinequalities between the threephases of the system can lead to some ofthese harmonics being presented to the load, just as inequalities intheprior art system could increase the amount of harmonies or DCcomponent supplied to the load. However, Applicant has found that a truel2-step voltage as shown in FIG. 8, waveform 68, is that which isapplied to the load.

The present disclosure includes that contained in the appended claims,as well as that of the foregoing description. Although this inventionhas been described in its preferred form with a certain degree ofparticularity, it is understood that the present disclosure of thepreferred form has been made only by way of example and that numerouschanges in the details of the circuit and the combination andarrangement of circuit elements may be resorted to without departingfrom the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

l. Aninverter circuit for supplying energy to a plurality of loads,comprising in combination,

a plurality of polyphase inverters operable at a minimum phasedisplacement therebetween of 360/2NP where N is the number of invertersand P is the number of phases of each inverter,

reactor means having a core with P legs,

winding means on said reactor means for each phase of each inverter,

and means connecting said winding means in series circuit with eachinverter phase and its respective load of a plurality of loads toestablish ampereturns which vectorially cancel the fundamental frequencycomponent of the flux within said reactor means and pass the fundamentalfrequency to the loads.

2. An inverter circuit as set forth in claim 1, wherein said connectingmeans connects said winding means between each inverter phase and itsrespective load.

3. An inverter circuit as set forth in claim 1, wherein said connectingmeans connects a load between each inverter phase and its respectivereactor phase.

4. An inverter circuit as set forth in claim 1, wherein said windingmeans includes the same number of turns on each phase to providecancellation of the fundamental frequency component of flux when theloads are equal.

5. An inverter circuit as set forth in claim 1, wherein the loads arepolyphase loads equal in number to the number of inverters.

' 6. An inverter circuit as set forth in claim 1, wherein the loads arepolyphase having the same number of phases as the phases of saidinverters.

7. An inverter circuit as set forth in claim 1, wherein said windingmeans includes a plurality of windings connected in series between eachinverter phase and its respective load.

8. An inverter circuit as set forth in claim 1, wherein said connectingmeans establishes a fundamental frequency voltage on said loads equal to100 percent of the fundamental frequency output voltage of each inverterdespite phase displacement of the outputs of said inverters.

9. An invertercircuit as set forth in claim 1, wherein said connectingmeans establishes the load power factor directly reflected to eachinverter.

10. An inverter circuit as set forth in claim 1, wherein said connectingmeans establishes the currents in said plurality of inverters with aphase displacement there between of 360l2NP.

11. An inverter circuit as set forth in claim 1, wherein said connectingmeans connects a plurality of winding means on each phase of saidreactor means to establish vector addition of the fundamental ampereturns on each phase to equal'zero when the plurality of loads are equal.

12. An inverter circuit as set forth in claim 1, wherein said reactormeans has a volt-ampere capacity only for harmonic voltages.

13. An inverter circuit as set forth in claim 1, wherein said connectingmeans connects said winding means to absorb within the reactor the NKPi1 harmonics present in said plurality of inverters to eliminate saidharmonies from the load, where K is any odd integer.

14. An inverter circuit as set forth in claim I, wherein said connectingmeans establishes said inverters as DC isolated in the load connections.

15. An inverter circuit as set forth in claim 14, including connectingthe DC inputs of said plurality of inverters in series on a single pairof DC supply terminals.

16. An inverter circuit as set forth in claim 1, wherein said reactormeans includes a single polyphase reactor.

17. An inverter circuit as set forth in claim 16, including windingmeans on each phase of said reactor corresponding to a phase of saidinverters.

18. An inverter circuit as set forth in claim 17, including two windingsin said winding means on each phase of said reactor.

19. An inverter circuit as set forth in claim 17, including a major anda minor winding on each phase of said reactor.

20. An inverter circuit as set forth in claim 19, including fourwindings on each phase of said reactor including two major and two minorwindings,

a major and a minor winding being energized from different phases of oneof the inverters and another major and minor winding being energizedfrom different phases of another of said inverters.

21. An inverter circuit as set forth in claim 20, in-

cluding a single rotor acted on by the plural loads.

22. An inverter circuit as set forth in claim 1, wherein said windingmeans consists of only one winding on each phase of said reactor meansfor one inverter and two windings on each phase of said reactor meansfor the other inverter.

23. An inverter circuit for supplying energy to a plurality of loads,comprising in combination,

a plurality of polyphase inverters operable at a minimum phasedisplacement therebetween of 360/2NP where N is the number of invertersand P is the number of phases of each inverter,

reactor means having a core with P legs,

winding means on said reactor means for each phase of each inverter,

means connecting said winding means in series circuit with each inverterphase and its respective load of a plurality of loads to establishampereturns which vectorially cancel the fundamental frequency componentof the flux within said reactor means and pass the fundamental frequencyto the loads,

and said connecting means establishing a voltage from said invertersapplied to the combination of said reactor means and loads which has anabsence of P harmonic voltage.

24. An inverter circuit as set forth in claim 23, wherein saidconnecting means connects a load between each inverter phase and itsrespective reactor phase.

25. An inverter circuit as set forth in claim 23, wherein said windingmeans includes a plurality of windings connected in series between eachinverter phase and its respective load.

26. An inverter circuit as set forth in claim 23, wherein saidconnecting means establishes a fundamental frequency voltage on saidloads equal to 100 percent of the fundamental frequency output voltageof each inverter despite phase displacement of the outputs of saidinverters.

27. An inverter circuit as set forth in claim 23, wherein saidconnecting means causes the load power factor to be directly reflectedto each inverter.

28. An inverter circuit as set forth in claim 23, wherein saidconnecting means establishes the currents in said plurality of inverterswith a-phase displacement therebetween of 360/2NP.

29. An inverter circuit as set forth in claim 23, wherein saidconnecting means connects a plurality of winding means on each phase ofsaid reactor means to establish vector addition of the fundamentalampere turns on each phase to equal zero when the plurality of loads areequal.

30. An inverter circuit as set forth in claim 23,

wherein said reactor means has a volt-ampere capacity only for harmonicvoltages. v

31. An inverter circuit as set forth in claim 23,

wherein said connecting means connects said winding means tosubstantially absorb within the reactor the NP i1 harmonics present insaid plurality of inverters to substantially eliminate said harmonicsfrom the load. 32. An inverter circuit as set forth in claim 23, whereinsaid connecting means connects said winding means to absorb within thereactor the NKP i1 harmonics present in said plurality of inverters toeliminate said harmonics from the load where K is any odd integer.

33. An inverter circuit as set forth in claim 23, including connectingthe DC inputs of said plurality of inverters in series on a single pairof DC supply terminals.

34. An inverter circuit as set forth in claim 23, wherein said reactormeans includes a single polyphase reactor.

35. An inverter circuit as set forth in claim 34, including a major anda minor winding on each phase of said reactor.

36. An inverter circuit as set forth in claim 35, including fourwindings on each phase of said reactor including'two major and two minorwindings,

a major and a minor winding being energized from different phases of oneof the inverters and another major and minor winding being energizedfrom different phases of another of said inverters.

1. An inverter circuit for supplying energy to a plurality of loads,comprising in combination, a plurality of polyphase inverters operableat a minimum phase displacement therebetween of 360*/2NP where N is thenumber of inverters and P is the number of phases of each inverter,reactor means having a core with P legs, winding means on said reactormeans for each phase of each inverter, and means connecting said windingmeans in series circuit with each inverter phase and its respective loadof a plurality of loads to establish ampere-turns which vectoriallycancel the fundamental frequency component of the flux within saidreactor means and pass the fundamental frequency to the loads.
 2. Aninverter circuit as set forth in claim 1, wherein said connecting meansconnects said winding means between each inverter phase and itsrespective load.
 3. An inverter circuit as set forth in claim 1, whereinsaid connecting means connects a load between each inverter phase andits respective reactor phase.
 4. An inverter circuit as set forth inclaim 1, wherein said winding means includes the same number of turns oneach phase to provide cancellation of the fundamental frequencycomponent of flux when the loads are equal.
 5. An inverter circuit asset forth in claim 1, wherein the loads are polyphase loads equal innumber to the number of inverters.
 6. An inverter circuit as set forthin claim 1, wherein the loads are polyphase having the same number ofphases as the phases of said inverters.
 7. An inverter circuit as setforth in claim 1, wherein said winding means includes a plurality ofwindings connected in series between each inverter phase and itsrespective load.
 8. An inverter circuit as set forth in claim 1, whereinsaid connecting means establishes a fundamental frequency voltage onsaid loads equal to 100 percent of the fundamental frequency outputvoltage of each inverter despite phase displacement of the outputs ofsaid inverters.
 9. An inverter circuit as set forth in claim 1, whereinsaid connecting means establishes the load power factor directlyreflected to each inverter.
 10. An inverter circuit as set forth inclaim 1, wherein said connecting means establishes the currents in saidplurality of inverters with a phase displacement therebetween of360*/2NP.
 11. An inverter circuit as set forth in claim 1, wherein saidConnecting means connects a plurality of winding means on each phase ofsaid reactor means to establish vector addition of the fundamentalampere turns on each phase to equal zero when the plurality of loads areequal.
 12. An inverter circuit as set forth in claim 1, wherein saidreactor means has a volt-ampere capacity only for harmonic voltages. 13.An inverter circuit as set forth in claim 1, wherein said connectingmeans connects said winding means to absorb within the reactor the NKP +or - 1 harmonics present in said plurality of inverters to eliminatesaid harmonics from the load, where K is any odd integer.
 14. Aninverter circuit as set forth in claim 1, wherein said connecting meansestablishes said inverters as DC isolated in the load connections. 15.An inverter circuit as set forth in claim 14, including connecting theDC inputs of said plurality of inverters in series on a single pair ofDC supply terminals.
 16. An inverter circuit as set forth in claim 1,wherein said reactor means includes a single polyphase reactor.
 17. Aninverter circuit as set forth in claim 16, including winding means oneach phase of said reactor corresponding to a phase of said inverters.18. An inverter circuit as set forth in claim 17, including two windingsin said winding means on each phase of said reactor.
 19. An invertercircuit as set forth in claim 17, including a major and a minor windingon each phase of said reactor.
 20. An inverter circuit as set forth inclaim 19, including four windings on each phase of said reactorincluding two major and two minor windings, a major and a minor windingbeing energized from different phases of one of the inverters andanother major and minor winding being energized from different phases ofanother of said inverters.
 21. An inverter circuit as set forth in claim20, including a single rotor acted on by the plural loads.
 22. Aninverter circuit as set forth in claim 1, wherein said winding meansconsists of only one winding on each phase of said reactor means for oneinverter and two windings on each phase of said reactor means for theother inverter.
 23. An inverter circuit for supplying energy to aplurality of loads, comprising in combination, a plurality of polyphaseinverters operable at a minimum phase displacement therebetween of360*/2NP where N is the number of inverters and P is the number ofphases of each inverter, reactor means having a core with P legs,winding means on said reactor means for each phase of each inverter,means connecting said winding means in series circuit with each inverterphase and its respective load of a plurality of loads to establishampere-turns which vectorially cancel the fundamental frequencycomponent of the flux within said reactor means and pass the fundamentalfrequency to the loads, and said connecting means establishing a voltagefrom said inverters applied to the combination of said reactor means andloads which has an absence of P harmonic voltage.
 24. An invertercircuit as set forth in claim 23, wherein said connecting means connectsa load between each inverter phase and its respective reactor phase. 25.An inverter circuit as set forth in claim 23, wherein said winding meansincludes a plurality of windings connected in series between eachinverter phase and its respective load.
 26. An inverter circuit as setforth in claim 23, wherein said connecting means establishes afundamental frequency voltage on said loads equal to 100 percent of thefundamental frequency output voltage of each inverter despite phasedisplacement of the outputs of said inverters.
 27. An inverter circuitas set forth in claim 23, wherein said connecting means causes the loadpower factor to be directly reflected to each inverter.
 28. An invertercircuit as set forth in claim 23, wherein said connecting meansestablishes the currents in said plurality of inverters with a phasedisplAcement therebetween of 360*/2NP.
 29. An inverter circuit as setforth in claim 23, wherein said connecting means connects a plurality ofwinding means on each phase of said reactor means to establish vectoraddition of the fundamental ampere turns on each phase to equal zerowhen the plurality of loads are equal.
 30. An inverter circuit as setforth in claim 23, wherein said reactor means has a volt-ampere capacityonly for harmonic voltages.
 31. An inverter circuit as set forth inclaim 23, wherein said connecting means connects said winding means tosubstantially absorb within the reactor the NP + or - 1 harmonicspresent in said plurality of inverters to substantially eliminate saidharmonics from the load.
 32. An inverter circuit as set forth in claim23, wherein said connecting means connects said winding means to absorbwithin the reactor the NKP + or - 1 harmonics present in said pluralityof inverters to eliminate said harmonics from the load where K is anyodd integer.
 33. An inverter circuit as set forth in claim 23, includingconnecting the DC inputs of said plurality of inverters in series on asingle pair of DC supply terminals.
 34. An inverter circuit as set forthin claim 23, wherein said reactor means includes a single polyphasereactor.
 35. An inverter circuit as set forth in claim 34, including amajor and a minor winding on each phase of said reactor.
 36. An invertercircuit as set forth in claim 35, including four windings on each phaseof said reactor including two major and two minor windings, a major anda minor winding being energized from different phases of one of theinverters and another major and minor winding being energized fromdifferent phases of another of said inverters.